tRNA transcription occurs in form of heteropolymer. the whole trancript has some tRNAs and some rRNAs that is further cleaved by specific mechanisms. the transcription of complete polymer ends most probably by secondary structure formation in the trancript. this termination is similar to the rho independent termination. I am not completely sure about it but i guess u can find this answer in any good molecular biology book. try J.D.Watson.